Is there any harm to mixing spark plugs?

I have 2 iridium plugs a buddy gave me, that I want to put into a V6. Any harm buying 4 plats or coppers and putting 2 iridiums in the other chambers?

 

Having 2 different kinds of plugs will cause the cylinders to burn slightly differently. In reality, the differences will be so small that you will probably never notice.

Is there ANY benefit from mix-matching plugs in your motor? No.

Would I run 2 different plugs? Not a chance in hell.

On the flip side, Toyota used to use 2 different BRANDS of plugs on some of their V6 engines, such as in the 4-Runner, Tacomas, etc. One bank of cylinders would have a different brand than the other bank. Obviously, the engineers knew something about the engine that warranted it, but I'll be damned if I know. Any time I change the plugs in one of those motors I run the same plugs in all 6 cylinders.

 

The more important question (besides asking how the ^%&$ you can't afford a complete set of plugs) is "Are these plugs (that your buddy gave you) the correct application for your ride."
IF they are a marked departure from the correct heat range, you can see issues ranging from fouling plugs (too cold) to detonation and burned pistons (too hot).
I would advise against it.

P